The Bible is clear. While we can't exhaustively comprehend it all, the divine authorial intention communicated through the human authors is suffuciently clear for all purposes of salvation (WCF 1.7).

But why is Scripture clear? Here's one of the most basic and solid arguments:

John Frame writes,

Since Scripture is God’s word, it is his communication to us. In Scripture, God speaks, not primarily to himself or to the angels, or to the winds and waves, but to us human beings. God cannot fail to accomplish his purpose, so his communication cannot be anything less than successful. If words are unclear, they fail to communicate; they are not communication. So Scripture must be clear.